The present invention relates to an artificial disc that does not include a joint or significant sliding portions, but which still maintains the flexibility of the spine, as well as the cushioning effect of the disc, after surgical replacement of an intervertebral disc. In more detail, the present invention relates to an artificial disc, intended for use in surgical replacement of an intervertebral disc, that retains the properties of cushioning and resistance to flexure of the spine, as well as allowing other normal range of motions, that characterize the healthy, normal intervertebral disc.
The injured, deformed, diseased, and/or degenerated human spine is a source of great pain in many patients, and there are many approaches to management, treatment, and/or prevention of that pain, including surgical intervention. One particularly vexing source of spinal pain and/or dysfunction is the damaged intervertebral disc. Healthy intervertebral discs are a necessity to pain-free, normal spinal function, yet disc function is all too frequently impaired by, for instance, disease or injury.
The anatomy of the intervertebral disc correlates with the biomechanical function of the disc. The three major components of the disc that are responsible for the function of the disc are the nucleus pulposus, annulus fibrosus, and cartilagenous endplate. The nucleus pulposus is the centrally located, gelatinous network of fibrous strands, surrounded by a mucoprotein gel, that prevents buckling of the annulus and maintains the height of the disc (and therefore, provides the cushioning effect and resistance to spinal flexure that are so important to spinal function) through osmotic pressure differentials. The water content of the disc changes in accordance with the load on the spine, water being driven out of the pulposus under heavy load. The annulus fibrosus encapsulates the disc, resisting both tension and compression loads and bearing axial loads. The vertebral endplates are cartilagenous in nature and “sandwich” the other components of the disc, distributing load over the entire disc and providing stability during normal spinal movements. The three elements work in cooperative fashion to facilitate disc function, and impairment of any of the elements compromises the functions of the other elements.
The two main surgical treatments of the intervertebral disc include total disc and nuclear replacement, but unfortunately, both treatments represent a number of compromises that simply do not provide normal disc function. The total artificial disc prosthesis is a total prosthetic replacement of the annulus fibrosus and nucleus pulposus with an endplate that interfaces with the patient's own vertebral endplates. Capturing and securing the total disc prosthesis to the host vertebral endplates can be a challenge because of the asymmetrical and cyclic loads placed upon the spine that can place excessive stresses on both the host bone and the interface between the prosthesis and the endplates, resulting in early loss of fixation. Many presently available total disc prostheses are designed to mimic the function of normal joints, but in that aspect, they are non-physiological in the sense that the normal spine does not have actual joints or sliding functions, but does have an inherent shock absorbing function. This lack of cushioning and shock absorbing function may be the contributing factor for the settling of the prosthesis into the vertebral body. For a summary of some of the disadvantages and limitations of known disc replacements, reference may be made to C. M. Bono and S. R. Garfin, History and Evolution of Disc Replacement, The Spine Journal, Vol. 4, pp. 145S-150S (2004) and E. G. Santos, et al., Disc Arthroplasty: Lessons Learned from Total Joint Arthroplasty, The Spine Journal, Vol. 4, pp. 182S-189S (2004).
Nuclear replacement is intended to replace a damaged nucleus pulposus with a device that is intended to restore disc height while maintaining the kinematics of the gel that comprises the healthy, intact nucleus pulposus. Although less invasive of the spine, implant extrusion and migration of the implant are all too frequent complications of nuclear replacement surgery. Some of the disadvantages and limitations of known devices for disc replacement are summarized in C. M. Bono and S. R. Garfin, History and Evolution of Disc Replacement, The Spine Journal, Vol. 4, pp. 145S-150S (2004) and in A. N. Sieber and J. P. Kostuik, Concepts in Nuclear Replacement, The Spine Journal, Vol. 4, pp. 322S-324S (2004).
It is, therefore, an object of the present invention to provide a total artificial intervertebral disc that is intended to overcome the disadvantages and limitations of these prior art devices comprising a frame, a cushion molded over the frame, and a screw passing through said frame and at least a portion of said cushion. An anchor is mounted to the screw for selectively engaging the vertebrae adjacent the intervertebral disc space when the frame, having the cushion molded thereover, is inserted into the space between two adjacent vertebrae and the screw is turned, thereby resisting anterior-posterior movement of the artificial disc relative to the adjacent vertebrae.
Another object of the present invention is to provide a total artificial disc that maintains the normal range of motion of the spine and provides a cushioning function that approximates the normal function of the intervertebral disc under compression load.
Another object of the present invention is to provide a total artificial disc that is comprised of three main components that together function to provide the cushioning provided by cooperation of the three components of the normal intervertebral disc.
Another object of the present invention is to provide a total artificial disc in which the axis of rotation translates in the anterior-posterior direction in a manner that approximates normal disc function.
Another object of the present invention is to provide a total artificial disc that is adapted for use in adjacent segments of the spine.
Another object of the present invention is to provide a frame for an intervertebral disc prosthesis comprised of two spaced apart, substantially parallel arms, a bridge connecting the arms at one end, a “U”-shaped ear extending at approximately a right angle from the end of one of the arms opposite the bridge and having a hole formed therein for receiving a screw, and a “Y”-shaped ear extending at approximately a right angle from the end of one of the arms opposite the bridge having holes formed in both forks of the Y-shaped ear for receiving screws, the frame being comprised of a material that tends to return to its original shape after the frame is subjected to either a compression or tension load.
Other objects, and the many advantages of the present invention, will be made clear to those skilled in the art in the following detailed description of several preferred embodiments of the present invention and the drawings appended hereto. Those skilled in the art will recognize, however, that the embodiments of the invention described herein are only examples provided for the purpose of describing the making and using of the present invention and that they are not the only embodiments of artificial discs that are constructed in accordance with the teachings of the present invention.